A conventional image forming device includes a conveyor belt for conveying a recording sheet and an image forming unit such as a process unit disposed in confrontation with a conveying surface of the conveyor belt for forming an image on a surface of the recording sheet conveyed by the conveyor belt. In the image forming device described above, if the recording sheet placed on the conveyor belt should be uniformly flat or smooth otherwise wrinkles are generated on the recording sheet causing lack of any part of the image formed on the recording sheet.
To solve this problem, U.S. Pat. No. 5,740,512 discloses an image forming device that has a guide member guiding the recording sheet from obliquely above to a conveying surface of conveyor belt. The guide member is provided with a projection protruding from a center portion of the guide member in a widthwise direction of the recording sheet toward the conveying surface of conveyor belt. Widthwise center region of the recording sheet is pressed by the projection toward the conveying surface of conveyor belt, and then, the center region of recording sheet is brought with contact with the conveying surface, and then, the contact with the conveying surface gradually proceeds toward the widthwise edge regions of the recording sheet. As a result, no air is involved or trapped between the conveying surface of the conveyor belt and the recording sheet when the recording sheet is placed on the conveying surface. Thus the recording sheet can be placed on the conveying belt with the surface of recording sheet being smoothed uniformly.
However, the inventor of the present application found degradation of output image in the image forming device disclosed in U.S. Pat. No. 5,740,512, if a plurality of recording sheets are stacked in highly humid condition (especially, in high-temperature and high-humidity condition) for long periods of time.
Specifically, in such highly-humid condition, a center region of each sheet maintains dried condition whereas an outline region thereof becomes moist. In other words, the center region has stiffness or linearity higher than that of the outline region.
Therefore, when such a recording sheet is conveyed by a roller, a center region of the recording sheet is conveyed ahead of the widthwise end regions in a conveying direction, due to the difference in stiffness, wrinkles may be generated in the widthwise end regions. In addition, when this recording sheet conveys into the guide member of the image forming device disclosed in U.S. Pat. No. 5,740,512, the preceding center region is pressed toward the conveying belt by the guide member and conveyed downstream prompting difference in sheet conveying manner between the center region and the widthwise end regions. Consequently, generation of wrinkles at the widthwise end regions is promoted or assisted. As a result, a gap is formed between the recording sheet and the conveying surface of conveyor belt due to the wrinkles when the recording sheet is placed on the conveyor belt, and the gap adversely affects image quality due to insufficient discharge of the air from the gap.